The optical lithography industry is currently transitioning to the use of very short exposure wavelengths for the purpose of reducing line weights (conductors, etc.) in integrated circuits, thereby to enhance performance of those circuits. In this regard, the next generation of optical lithography systems will use laser light having a wavelength of about 157 nanometers, which wavelength is often referred to as deep ultraviolet or DUV.
It is important to precisely determine the optical characteristics of the optical elements that are used in systems that employ DUV light. Such an element may be, for example, a calcium fluoride (CaF2) lens of a scanner or stepper. Birefringence is one such characteristic of the optical element.
Birefringence is an intrinsic property of many optical materials, and may also be induced by external forces. The terms retardation or retardance represent the integrated effect of birefringence acting along the path of a light beam traversing a sample optical element. If the incident light beam is linearly polarized with the direction of polarization different from the fast axis of the sample, the two orthogonal components of the polarized light will exit the sample with a phase difference, called the retardance. The unit of retardance can be length, such as nanometers (nm). It is also frequently expressed in units of phase angle (waves, radians, or degrees), which angle is directly proportional to the retardance (nm) divided by the wavelength of the light (nm). A path “average” birefringence for a sample is sometimes computed by dividing the measured retardation magnitude by the thickness of the sample. Oftentimes, the term “birefringence” is interchangeably used with and carries the same meaning as the term “retardance.” Thus, unless stated otherwise, those terms are also interchangeably used below.
Since the retardance of an optical element is a characteristic of both the optical material and the wavelength of the light that penetrates the material, a system for measuring retardance properties (hereafter usually referred to as a birefringence measurement system) of an optical element employed in a DUV optical setup must also operate with a DUV light source and associated components in order to precisely detect and process the associated light signals.
There are several problems associated with the use of DUV light in applications such as birefringence measurement or photolithography. One problem concerns absorption of DUV light by oxygen present in the system environment, and in the light beam path in particular. In this regard, the oxygen molecules (as well as other contaminants such as water vapor or trace amounts of hydrocarbons) absorb the DUV light, thus attenuating the light and reducing the signal necessary to make accurate birefringence measurements of the sample.
One way of eliminating the oxygen (as well as other contaminants) in the system environment is to purge the system with nitrogen (N2). Purging, and the maintenance of a purged system, however, will often require reductions in throughput or large, expensive purging systems, especially in instances where a large number of optical components are involved, or the equipment incorporating the optical elements is large.